1. Field of the Invention
The present invention relates to a method of manufacturing such a type of liquid crystal display device that liquid crystal is controlled by electric field applied in a direction parallel to a substrate, and particularly to a method of manufacturing a liquid crystal display device which can control abnormality of orientation of the liquid crystal in the neighborhood of a common electrode and a picture element electrode (pixel electrode) of a unit picture element portion (unit pixel portion) to prevent reduction of contrast and also enhance the image quality.
2. Description of the Related Art
A TN (Twisted Nematic) type liquid crystal display device is known as one representative type of liquid crystal display devices for controlling liquid crystal by applying electric field in the direction vertical to a substrate (in the longitudinal direction). The TN type liquid crystal display device includes: a first transparent glass substrate having gate bus lines and drain bus lines, on which active elements and pixel electrodes are arranged in a matrix form at the respective intersection points between the gate bus lines and the drain bus lines; a second transparent glass substrate having a colored layer of R (red), G(green) and B(blue), a light shielding film and a common electrode; and a liquid crystal layer interposed between the first and second glass substrates. In such a TN type liquid crystal display device, the liquid crystal is controlled by electric field which is generated by applying an electric voltage between each of the pixel electrodes and the common electrode.
Besides, there is also known an IPS (in-plane switching) type of liquid crystal display device which is one representative type for controlling the liquid crystal by applying electric field in the direction parallel to the substrate (in the lateral direction). The IPS type liquid crystal display device includes: a first transparent glass substrate having gate bus lines and drain bus lines, on which active elements and interdigitally-arranged pixel electrodes and common electrodes are arranged in a matrix form at the respective intersection points between the gate bus lines and the drain bus lines; a second transparent glass substrate having a colored layer of R(red), G(green) and B(blue) and a light shielding film; and a liquid crystal layer disposed between the first and second transparent glass substrates. In this IPS type liquid crystal display device, the liquid crystal is controlled by applying a voltage between each pixel electrode and the common electrode, and thus electric field in the lateral direction is used.
A method of manufacturing the conventional IPS type liquid crystal display device will be described with reference to the accompanying drawings. FIG. 1 is a plan view showing a unit pixel portion of the conventional IPS type liquid crystal display device. FIG. 2A is a detailed diagram showing a rubbing processing direction in the unit pixel portion, and FIG. 2B is a cross-sectional view of FIG. 2A.
In a conventional process of manufacturing the IPS type liquid crystal display device, by performing a photolithography process at plural times, gate bus lines 37, drain bus lines 38 and active elements 39 are formed in a matrix form on a transparent glass substrate 36, and a common electrode 41 and a pixel electrode 42 are formed in a desired pattern of an interdigital arrangement in each unit pixel portion. Subsequently, as shown in FIG. 2B, by using an offset printing method or the like, an orientation film 45 is formed on the transparent glass substrate 36 on which the bus lines 37, 38, the active elements 39 and the electrodes 41 and 42 are formed, and then the surface of the orientation film 45 is subjected to a rubbing treatment which specifies the orientation direction of the liquid crystal molecules. As disclosed in Japanese Laid-open Patent Application No. Hei-7-306417, it is preferable that the rubbing treatment is performed in a direction 53 which traverses obliquely the interdigital arrangement of the common electrode 41 and the pixel electrode 42 as shown in FIG. 2A, that is, at an intersection angle of about 1 degree to 20 degrees to the normal direction of the electric field.
In the process of the liquid crystal display device, the interdigitally-arranged common electrode 41 and pixel electrode 42 in each unit pixel portion are designed so that the electrode width of each electrode is set to about 5 micron, the inter-electrode space is set to about 10 microns, about six pairs of electrodes are formed in each pixel portion and the film thickness of each electrode is set to 1000 to 2000 angstroms, so that a shady portion occurs in the vicinity of each electrode in the unit pixel portion due to the film thickness of the electrode, that is, due to an electrode step. Therefore, in the rubbing process, there occurs an area 60 (rubbing non-contact area) which has no or little orientation restricting force because in the above shady portion the tip of rubbing cloth is not or extremely weakly brought into contact with the orientation film 45 so that the orientation film 45 is not subjected to the rubbing treatment or subjected to the rubbing treatment with the rubbing strength of extremely weaker than that in other areas. If such an area 60 having no or little orientation restricting force exists in the orientation film 45 between the electrodes, the orientation direction of the liquid crystal molecules is not controlled to be uniform over the orientation film 45 between the electrodes, resulting in occurrence of an orientation abnormality in display. Therefore, the contrast is reduced, and hue is varied in accordance with a screen viewing angle to degrade the image quality.
A cause of the reduction in contrast as described above is as follows. That is, in the IPS system in which the control of the liquid crystal is performed by the electric field in the lateral direction, domains (reversely rotating domains) in which the liquid crystal molecules are rotated in the direction opposite to the normal rotational direction, etc. occur in the vicinity of the common electrodes 41 and the pixel electrodes 42 interdigitally-arranged in the unit pixel portion, and these domains appear as bright lines. Therefore, the brightness when displaying black is made higher, resulting in reduction of the contrast. Further, a cause of the reduction in image quality is as follows. If the viewing angle to the display plane exceeds 60 degrees in the vertical direction or in the right-and-left direction, a display looks colored even when the display is a white frame.
Therefore, means of solving the above problem has been required, however, it is the present situation for the IPS type liquid crystal device that no effective solving means has been supplied for the reduction of the image quality, etc. caused by the abnormality of orientation of the liquid crystal in the vicinity of the electrodes in the unit pixel portion.
On the other hand, in the case of the TN type liquid crystal display device, Japanese Laid-open Patent Application No. Hei-4-195119 discloses a method of carrying out the rubbing treatment at plural times in the forward and backward directions alternately while reducing the rubbing intensity to solve the problem of the reduction of the image quality, etc. which is caused by the occurrence of the rubbing non-contact area. However, in the case of this method, any previous rubbing history is not left, and as a result it is equivalent to a rubbing treatment which is merely carried out in the direction when the last rubbing. Therefore, the orientation control in the vicinity of the electrodes cannot be performed. As other methods have been proposed a method of designing the four corner portions of the pixel electrode in an arcuate form (Japanese Laid-open Patent Application No. Hei-4-285917), and a method of irradiating ultraviolet rays after rubbing to reduce the pre-tilt angle of the liquid crystal if the liquid crystal of the liquid crystal display device is limited to ferroelectric liquid crystal (Japanese Laid-open Patent Application No. Hei-7-333619). In any method, a manufacturing apparatus must be designed in a large scale, and also it is difficult to form a pattern having arcuate corner portions is complicated and to control the irradiation amount of ultraviolet rays, so that the manufacturing process is very difficult. Further, a method of laminating liquid crystal cells (Japanese Laid-open Patent Application No. Hei-2-204725, for example) is proposed as another method. However, in this method, transmittance is reduced to the half, so that the brightness of back light must be increased to two times and the power consumption is increased. Therefore, it is difficult to practically use this method.